An optical fiber gyro using an optical fiber which detects an angular velocity of a rotating member is widely used to control a car or vehicle body. In an optical fiber gyro using the phase modulation system, a light supplied from a light source is divided into two lights by an optical coupler and the two lights are shifted in phase to be supplied to an optical fiber sensing loop by a phase modulator. Then, the lights which are propagated through the sensing loop in the clock and counter clock-wise directions are coupled to be supplied to an optical detector by the optical coupler. In the optical detector, an electric signal is generated from the received light to detect an angular velocity of the sensing loop. Thus, a displacement angle of the optical fiber gyro is detected in accordance with the angular velocity.
The optical fiber gyro comprises a signal processing circuit which comprises a synchronous detector, an analog to digital (A/D) converter, a CPU (Central Processing Unit), and an oscillator.
In calculating an angular velocity, a sine-wave signal having a frequency fm is applied to the phase modulator by the oscillator. Thus, the clock-wise and counter clock-wise direction lights are phase-modulated in the sensing loop. As described before, an electric signal is generated in the optical detector. The electric signal comprises a DC component, a component of the phase-modulation frequency, and components of high harmonic waves to be supplied to the synchronous detector.
Here, the equation (1) is defined. ##EQU1## where P is an instantaneous output of the electric signal, P.sub.L and P.sub.R are outputs of the clock-wise and counter clock-wise direction lights, m is a phase-modulation degree, J.sub.o (m) to J.sub..o slashed. (m)are Bessel functions having a factor of m, and .o slashed., is a Sagnac phase difference.
Each frequency component is synchronously detected in the synchronous detector to be converted from analog value to digital value in the A/D converter. Then, the digital values of each frequency component are supplied to the CPU, in which an angular velocity .OMEGA. is calculated.
Where the synchronous detector comprises a switch which is turned on and off by a synchronous signal having a predetermined frequency, and an output voltage obtained from a low-pass filter which is included in the synchronous detector will be a maximum value at the time when an input signal supplied to the synchronous detector is coincident in phase with the synchronous signal. On the other hand, when the input signal is not coincident in phase with the synchronous signal to provide a phase difference .DELTA..theta., the output signal will not be the maximum value, while an error occurs in the output signal.
The equations (2) to (4) represent errors generated in the fundamental wave component S.sub.1, the duplicate wave component S.sub.3, and the quadruple wave component S.sub.4 in accordance with the phase difference .DELTA..theta.. EQU S.sub.1 =K.multidot.J.sub.1 (m) sin .o slashed.s.multidot.cos .DELTA..theta.(2) EQU S.sub.2 =K.multidot.J.sub.2 (m) cos .o slashed.s.multidot.cos (2.multidot..DELTA..theta.) (3) EQU S.sub.4 =K.multidot.J.sub.4 (m) cos .o slashed.s.multidot.cos (4.multidot..DELTA..theta.) (4)
where K is a constant which is determined by a light output and an amplification factor in an electric circuit.
In the conventional optical fiber gyro, an error occurs in the calculation of an angular velocity .OMEGA., if an error caused by the phase differences .DELTA..theta. is included in an output signal of the synchronous detector. Therefore, it is required to make the error caused by the phase difference a .DELTA..theta. zero, so that the precision becomes high in measuring an angular velocity .OMEGA..
In order to make the error zero, a synchronous detection circuit using two-phase synchronous signals comprising two signals of the same frequency and a different phase of .pi./2 (one quarter period) is adopted to detect a fundamental wave signal S.sub.1 ' as defined by the equation (5). EQU S.sub.1 '=K.multidot.J.sub.1 (m) sin .o slashed.s.multidot.sin .DELTA..theta. (5)
Then, a calculation is carried out as defined by the equation (6). ##EQU2##
On the other hand, a phase of a synchronous signal is adjusted in a phase-adjusting apparatus using a one-phase synchronous signal by delaying a phase of the synchronous signal via a delay circuit. In this phase-adjusting apparatus, a phase is adjusted to maximize a fundamental wave signal.
In the conventional phase-adjusting apparatus in the optical fiber gyro, however, there are disadvantages as described below.
(1) When the synchronous detection circuit using the two-phase synchronous signals is adopted, high harmonic wave components are included in the synchronously detected signal, as seen in the equations (2) and (6) by sin.DELTA..theta. and cos.DELTA..theta., so that an output error can not be completely eliminated.
(2) When the synchronous detection circuit using the two-phase synchronous signals is adopted, the scale of the synchronous detection circuit becomes twice to result in the enlargement of the size, and the cost thereof becomes high.
(3) When a phase of a synchronous signal is adjusted to maximize a synchronously detected signal, an output signal varies at the time when any rotation is applied to the sensing hoop, as apparent from the equations (2) to (4). Therefore, it is necessary for the sensing loop to be static in adjusting the phase of the input signal. This means that the phase adjustment is impossible to be carried out in operation of the optical fiber gyro.
(4) Even if the synchronous detection circuit using the two-phase synchronous signals ie adopted, a phase-adjusting circuit becomes necessary to some extent, because any rotation is detected in accordance with positiveness and negativeness of fundamental wave signals.